Missionary work is the proclamation of the gospel.... period. There is no specific command to "plant churches". However, IF there is a genuine interest among those who have heard the gospel and responded in a genuine repentance and faith, then the planting of a church would certainly be encouraged.
The Western World is the largest missionary field in existence at this time. There is much religiosity and/or spirituality and churches which are unbiblical despite their outward appearance. Thus, doing missionary work in one's own country is certainly legitimate.
Lastly, supporting missionaries whose work has not been fruitful should be required to move on and do their gospel work in another area. This is undeniably the precedent set by the apostles and disciples in Scripture. Let me add that missionaries need to be closely scrutinized particularly in their methods and message. Far too many are not using bibilically established standards. Consequently, because $$$$$$$$$$ is typically dependent upon the number of alleged converts, all forms of false methods and false gospel preaching is adopted in order to "win souls" by missionaries in order to retain their "job". Due to the current horrid state of the modern church, false conversions are the norm across denominational lines.
Matthew 7:13-15 (ASV) "Enter ye in by the narrow gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many are they that enter in thereby. For narrow is the gate, and straitened the way, that leadeth unto life, and few are they that find it. Beware of false prophets, who come to you in sheep's clothing, but inwardly are ravening wolves."